This invention relates to a system for drilling under-balanced wells. More specifically, but not by way of limitation, this invention relates to an apparatus and method for drilling wells with an under-balanced hydrostatic head of pressure.
In the search for hydrocarbons, a well is drilled to a subterranean reservoir containing hydrocarbons. As those of ordinary skill in the art appreciate, drilling fluids are placed into the well in order to control reservoir pressure. Uncontrolled pressure releases from subterranean reservoirs can lead to blow-outs, which in turn can lead to severe personal injury and property damage.
A very well known technique for controlling well pressure is to weight the drilling fluid. Hence, the weighted drilling fluid creates a sufficient pressure to prevent reservoir pressure from entering the well. However, in the cases where the subterranean reservoir is under pressured, or normally pressured, or in the event that the drilling fluid is over-weighted, the drilling fluid may migrate into the formation. The migration of drilling fluid may cause damage to the formation, and in particular, may cause permeability and skin damage to the formation, which in turn will lead to a loss of productivity and commercial feasibility.
A prior art technique is to drill the well in an under-balanced state. Special equipment, drilling fluids and operating techniques exist so that it is possible to drill with a drilling fluid to achieve a hydrostatic pressure below the formation pore pressure. This may mean that formation fluids and gas flow into the well while drilling.
In one prior art technique, a special tool is added on top of the BOP, called a rotating control head. This seals on the drill pipe while allowing rotation and movement of the drill string. It is necessary to do this so that the actual pressure at the bottom of the hole (drilling fluid hydrostatic pressure plus surface pressure plus formation pressure) can be controlled.
Despite these advances, another prior art technique has been developed which employs concentric tubulars. The concentric strings allow for a gas or air to be injected into a common annulus which in turn allows for safe mixing of gas or air and fluid while still maintaining control of the operation. Nevertheless, problems with slugging and well control are encountered as the drilling string advances and the gas or air is injected all the way down the conveying conduit and back up the bottom of the concentric casing string into the concentric casing return annulus. Some of the disadvantages of these prior art systems include exerting the injecting gas, air or fluid injection pressure on the open formation which causes slugging, formation damage and well bore stability problems. Also, the prior art systems are unable to regulate the amount of gas, air or fluid that is mixed with the return drilling fluid which causes slugging and hole cleaning issues of piled-up drill cuttings in the well bore. Therefore, there is a need for an apparatus and method for drilling under-balanced wells that can address these and other issues.